If
you are going to do a solar project, you need to do a Solar Site Survey to
make sure that your site gets enough sun during the right times of year to
make the project work.
This free and simple survey only takes an hour and is fun and educational.

The
free Skyview app is normally used to allow iPhones etc. to find stars in the
night sky, but it can also be used to do a first cut solar survey to find
obstacles that shade your solar collectors.

To use it, you make sure it is set to your location, then set the date and
time of interest, then select the sun object. Standing at your your
proposed collector location, pan across the horizon -- the path of the sun
will be projected on the screen so you can see when it is lower in the sky
than nearby trees, buildings etc. The logical first date
to start with would be Dec 21 (the winter Solstice) when the sun is lowest.

I tried this on my iPad, and it worked pretty well, but I would still do the
site survey listed just above rather than relying fully on Skyview -- it can
be a real disaster to get the shading wrong.

This
is a nice application from Andrew for the iPhone that allows you to do a
solar site survey using the iPhone's built in sensors for measuring
elevations and azimuth angles for the survey of the horizon.
While I've not had a chance to actually try it, it looks to me as though it
would do a good job.

The
Solar Pathfinder has been around for many years, and is used by many solar
professionals to do solar site surveys. Basically its a glass dome,
and the reflection of the objects on the horizon is used to assess the
effect of objects on the horizon that would block the sun. Now offered
with software assistant.
Probably a bit of overkill for the DIYer building one or two projects.

Bright
Harvest offers a new kind of solar survey. Based on aerial photos,
they make a digital model of your house and surrounding obstacles, and then
try various arrangements of collectors and estimate the shading for each.

This
very good paper provides a very simple set of contour plots that show optimum
collector orientation, and the performance effect for deviations from
optimal.
Covers only the US, but the general relationships would be useful elsewhere.
Some limitations -- read the cautions in the paper.

The very useful PVWatts tool can be used to work out the best orientation
for solar PV or thermal collectors at your location.
Simply try different tilt and azimuth angles until you maximize solar
irradiance on your panels over the desired time frame.

Climate
Consultant is one of the UCLA Energy Design Tools.
It provides a variety of ways to visualize weather for a specific location.
Weather data files are available for hundreds of locations. This is a
brand new version, and is very easy to use.
"It graphically displays climate data in either metric or imperial units in
dozens of ways useful to architects including monthly bar charts, timetable
charts, and psychrometric charts, sun shading charts, and sun dial charts."One of the most interesting charts is the Psychrometric chart
with overlays that show the design strategies (e.g. passive solar,
evaporative cooling, ..) will work for the location in question.

The CLIM20 product provides pretty detailed
historical weather summaries for many US towns. It includes
temperatures, precip, and heating and cooling degree days. Lots of
good data.

NOAA year to date degree days provides deg day totals for the current
year. Updated each week.

NREL Redbook:
Average weather conditions and solar radiation
of collectors at various orientations for many US cities. Based on 30
year weather records. Try downloading the pdf version for your state.
Can be used to roughly calculate gain for collectors.

NREL Bluebook: The Solar Radiation Manual for Buildings gives total and
diffuse radiation on S, N, E, W and horizontal surfaces.
This can be used as base data for many passive gain calculations.

Note that
PVWatts, which is a PV oriented calculator by NREL also provides solar
irradiance on the surface you specify, and this can be used as input for
solar thermal calculations in the same way.

This
is a very easy to use calculator that gives solar radiation levels for many
locations worldwide by month. It provides radiation levels for equator
facing surfaces, and allows you to pick from several tilt angles.

This
is really a tool for astronomers, but if you are a solar energy nut and you
are always obsessing over whether the next 48 hours are going to have any
good sun, this tool can be handy.
It gives a cloud cover forecast by hour for about 48 hours for thousands of
locations.

A
nice set of free web tools that give heating and cooling degree day
information for 1200 locations.
Provides historic, current, and even near term degree day forecasts.
Very easy to use.Note that a default balance point temperature is 55F is used -- you
can change it to the more usual 65F if you wish.

SketchUp
is a free 3D drawing tool from Google. In addition to being
easy to use as a general purpose drawing tool, it offers a
built in sun.
You can easily draw a wall with a window and overhang (or a whole home),
and play the sun over it for various times of day and times of year.
Shading structures of all types can be modeled, including trees, solar
sails, shade walls, window fins, ...
Sun patterns inside the home are also shown.
It would be good for modeling sunspaces and greenhouses as well.